Transmitting system for small-sized vehicle

ABSTRACT

In a transmitting system for a small-sized vehicle in which a crankshaft of an engine and an input shaft of a multi-stage transmission are connected to each other through a fluid transmitting device, a shifting clutch is interposed between the crankshaft of the engine and the multi-stage transmission in a series relation to the fluid transmitting device. A lock-up clutch is interposed between a pump impeller and a turbine impeller of the fluid transmitting device. Thus, when the multi-stage transmission is to be shifted, the shifting operation can be conducted lightly, in spite of a creep phenomenon of the fluid transmitting device, by bringing the shifting clutch into an OFF state. Moreover, during cruising of the vehicle, the slipping of the fluid transmitting device can be inhibited by the OFF state of the lock-up clutch to enhance the transmitting efficiency.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a transmitting system for asmall-sized vehicle, in which a crankshaft of an engine and an inputshaft of a multi-stage transmission are connected to each other througha fluid transmitting means including a pump impeller connected to theengine, and a turbine impeller connected to the multi-stagetransmission, i.e., through a torque converter or a fluid coupling.

[0003] 2. Description of the Related Art

[0004] Such transmitting system for small-sized vehicles is alreadyknown, as disclosed in, for example, Japanese Patent ApplicationLaid-open No.57-69163.

[0005] In such known transmitting system, as disclosed in the abovePublication, the crankshaft of the engine and the input shaft of themulti-stage transmission are connected to each other only through thetorque converter, so that a torque shock generated at the time ofstarting the vehicle or during shifting is absorbed by a slipping actionof the torque converter.

[0006] However, the known transmitting system suffers from the followingdrawbacks: The torque converter or the fluid coupling has a slippingfunction, but performs the transmission of a torque to certain degree,as long as power is input from the engine to the torque converter or thefluid coupling. Therefore, in the known system, at the time of startingthe vehicle in which the transmission is switched over from a neutralposition to a low or first-speed position, a creep phenomenon isproduced in which power is transmitted to a driven wheel of the vehicleto certain degree, even if the engine is in an idling state. Duringtraveling of the vehicle, the friction always occurs in switching andsliding portions of the transmission due to the transmitted torque. Forthis reason, there are inconveniences that the resistance to theswitching of the transmission is large, and a large shifting load isrequired.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to providea transmitting system for a small-sized vehicle above-described, whereinthe creep phenomenon is eliminated, and the shifting operation of thetransmission can be carried out lightly, and moreover, during cruisingof the vehicle, the slipping of the fluid transmitting means isinhibited to enhance the transmitting efficiency.

[0008] To achieve the above object, according to a first aspect andfeature of the present invention, there is provided a transmittingsystem for a small-sized vehicle in which a crankshaft of an engine andan input shaft of a multi-stage transmission are connected to each otherthrough a fluid transmitting means including a pump impeller connectedto the engine, and a turbine impeller connected to the multi-stagetransmission, wherein the transmitting system includes a shifting clutchwhich is interposed between the crankshaft of the engine and the inputshaft of the multi-stage transmission, the shifting clutch being in aseries relation to the fluid transmitting means, and a lock-up clutchwhich is interposed between the pump impeller and the turbine impellerof the fluid transmitting means, the lock-up clutch being capable ofconnecting both the impellers directly to each other.

[0009] The fluid transmitting means corresponds to a torque converter Tin each of embodiments of the present invention which will be describedhereinafter.

[0010] With the first feature, during idling of the engine, thetransmitting of power to the shifting clutch and the like can be cut offby controlling the shifting clutch to its OFF state irrespective of thepresence of the fluid transmitting means, even in a low or first-speedposition of the transmission, thereby preventing the creep phenomenon.During shifting, the transmission can be brought into an unloaded stateirrespective of the presence of the fluid transmitting means by firstcontrolling the shifting clutch to its OFF state, thereby conducting theshifting lightly without generation of a torque shock.

[0011] Moreover, during cruising of the vehicle, if the lock-up clutchis controlled to its ON state, the pump impeller and the turbineimpeller are connected directly to each other and hence, the slippingbetween both the impellers can be eliminated to prevent a loss of power.

[0012] According to a second aspect and feature of the presentinvention, in addition to the first feature, the lock-up clutchcomprises a pump extension connected to the pump impeller andsurrounding the turbine impeller, a pressure receiving plate coupled toa tip end of the pump extension to define, within the pump extension, ahydraulic pressure chamber communicating with an oil chamber definedbetween the pump impeller and the turbine impeller, a pressing plateopposed to the pressure receiving plate and biased toward the pressurereceiving plate by a hydraulic pressure in the hydraulic pressurechamber, an annular friction clutch plate interposed between thepressure receiving plate and the pressing plate and connected to theturbine impeller, first and second valve bores provided in the pressingplate and the pressure receiving plate on the side of an inner peripheryof the friction clutch plate, respectively, a control valve provided inthe pressing plate to close the first valve bore, a control rod which isreceived in the first and second valve bores and movable between aretracted position in which the control rod causes the inner peripheryof the friction clutch plate to be open outside the second valve bore,while permitting the closing of the control valve, and an advancedposition in which the control rod causes the control valve to be open topermit the inner periphery of the friction clutch plate to communicatewith the hydraulic pressure chamber, while closing the second valvebore, and an operating means for operating the control rod.

[0013] The operating means corresponds to a lock-up clutch operatingshaft 86 in a first embodiment of the present invention which will bedescribed hereinafter.

[0014] With the second feature, if the control rod is operated to theretracted position, the pressing plate can clamp the friction clutchplate between the pressing plate and the pressure receiving plate underthe action of a hydraulic pressure transmitted from the fluidtransmitting means to the hydraulic pressure chamber, whereby thelock-up clutch can be brought into its ON state. If the control rod isoperated to the advanced position, the hydraulic pressure in thehydraulic pressure chamber can be applied to opposite sides of thepressing plate to eliminate the clamping force to the friction clutchplate, whereby the lock-up clutch can be brought into its OFF state. Inthis OFF state, the control rod closes the second valve bore and hence,the useless leakage of the hydraulic pressure from the hydraulicpressure chamber can be prevented.

[0015] According to a third aspect and feature of the present invention,in addition to the first or second feature, a one-way clutch isinterposed between the crankshaft and the turbine impeller and broughtinto its ON state, when the turbine impeller receives a reverse loadtorque.

[0016] With the third feature, at the time of an engine brake duringtraveling of the vehicle, the one-way clutch is brought into its ONstate by application of the reverse load torque to the turbine impeller.Therefore, the turbine impeller and the crankshaft are connecteddirectly to each other, and the reverse load torque is transmitted tothe crankshaft without via the fluid transmitting means and thus, a goodengine brake effect can be provided.

[0017] According to a fourth aspect and feature of the presentinvention, in addition to the first feature, the lock-up clutch isconstructed such that it depends on the rotational speed of the pumpimpeller and it automatically operates, when the rotational speed of thepump impeller becomes equal to or higher than a predetermined value.

[0018] With the fourth feature, when the rotational speed of the pumpimpeller becomes equal to or higher than the predetermined value, thepump impeller and the turbine impeller of the fluid transmitting meanscan be automatically connected directly to each other.

[0019] According to a fifth aspect and feature of the present invention,in addition to the fourth feature, the lock-up clutch comprises a pumpextension connected to the pump impeller and surrounding the turbineimpeller, a pressure receiving plate coupled to a tip end of the pumpextension to define, within the pump extension, a hydraulic pressurechamber communicating with an oil chamber defined between the pumpimpeller and the turbine impeller, a pressing plate opposed to thepressure receiving plate for advancing and retracting movements, anannular friction clutch plate interposed between the pressure receivingplate and the pressing plate and connected to the turbine impeller, areturn spring for biasing the pressing plate in a direction to beretracted relative to the pressure receiving plate, and an escape borewhich permits the communication between the inside and outside of thepressure receiving plate on an inner peripheral side of the frictionclutch plate, wherein when the rotational speed of the pump impellerbecomes equal to or higher than the predetermined value, the pressingplate clamps the friction clutch plate in cooperation with the pressurereceiving plate under the action of a centrifugal hydraulic pressure inthe hydraulic pressure chamber which is raised in accordance with therotational speed of the pump impeller.

[0020] With the fifth feature, the automatic control of the lock-upclutch depending on the rotational speed of the pump impeller can beconducted easily.

[0021] According to a sixth aspect and feature of the present invention,in addition to the first feature, the lock-up clutch is constructed suchthat it depends on the rotational speed of the turbine impeller and itoperats automatically when the rotational speed of the turbine impellerbecomes equal to or higher than a predetermined value.

[0022] With the sixth feature, when the rotational speed of the turbineimpeller becomes equal to or higher than the predetermined value, thepump impeller and the turbine impeller of the fluid transmitting meanscan be automatically connected directly to each other.

[0023] According to a seventh aspect and feature of the presentinvention, in addition to the sixth feature, the lock-up clutchcomprises a clutch cylinder connected to the turbine impeller, apressing piston slidably received in a cylinder bore in the clutchcylinder to define a hydraulic pressure chamber, a piston return springfor biasing the pressing piston toward the hydraulic pressure chamber, ameans for introducing the oil into the hydraulic pressure chamber, and afriction engagement means provided between the clutch cylinder and thepump impeller, wherein when the rotational speed of the turbine impellerbecomes equal to or higher than the predetermined value, the pressingpiston operates the friction engagement means under the action of acentrifugal hydraulic pressure within the hydraulic pressure chamberwhich is raised in accordance with the rotational speed of the turbineimpeller to connect the clutch cylinder and the pump impeller directlyto each other.

[0024] The friction engagement means corresponds to driving frictionclutch plates 110, driven friction clutch plates 111 and transmittingclaws 112 in a third embodiment of the present invention which will bedescribed hereinafter, and the oil introducing means corresponds to theinlet bore 117 in the third embodiment.

[0025] With the seventh feature, the automatic control of the lock-upclutch depending on the rotational speed of the turbine impeller can becarried out easily.

[0026] According to an eighth aspect and feature of the presentinvention, in addition to the seventh feature, the clutch cylinder isprovided with an escape bore which opens an outer periphery of thehydraulic pressure chamber to the outside, and a centrifugal valve whichopens the escape bore, when the rotational speed of the clutch cylinderis lower than a predetermined value, and closes the escape bore, whenthe rotational speed of the clutch cylinder is equal to or higher thanthe predetermined value.

[0027] With the eighth feature, when the rotational speed of the clutchcylinder is lower than the predetermined value, the pressure remainingin the hydraulic pressure chamber can be released promptly through theescape bore by opening of the centrifugal valve to enhance theturning-off performance of the lock-up clutch, and a foreign matter suchas a cut powder within the hydraulic pressure chamber can be dischargedthrough the escape bore along with the oil. When the rotational speed ofthe clutch cylinder is equal to or higher than the predetermined value,the rising of the hydraulic pressure in the hydraulic pressure chambercan be conducted by closing of the centrifugal valve, and the operationof the lock-up clutch cannot be impeded.

[0028] The above and other objects, features and advantages of theinvention will become apparent from the following description of thepreferred embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIGS. 1 to 12 show a first embodiment of the present invention,wherein

[0030]FIG. 1 is a side view of a motorcycle to which the presentinvention is applied;

[0031]FIG. 2 is a vertical sectional view of a power unit mounted in themotorcycle;

[0032]FIG. 3 is an enlarged vertical sectional view of a transmittingsystem in the power unit;

[0033]FIG. 4 is a sectional view taken along a line 4-4 in FIG. 3;

[0034]FIG. 5 is a view taken along a line 5-5 in FIG. 3;

[0035]FIG. 6 is a side view of the transmitting system;

[0036]FIG. 7 is an enlarged view showing an outlet valve in a shiftingclutch in a closed state in FIG. 3;

[0037]FIG. 8 is an enlarged view showing the outlet valve in an openedstate;

[0038]FIG. 9 is a sectional view taken along a line 9-9 in FIG. 3;

[0039]FIG. 10 is a sectional view taken along a line 10-10 in FIG. 3;

[0040]FIG. 11 is an enlarged view showing a control valve in a lock-upclutch in a closed state in FIG. 3;

[0041]FIG. 12 is an enlarged view showing the control valve in an openedstate;

[0042]FIG. 13 is a sectional view similar to FIG. 3, but according to asecond embodiment of the present invention;

[0043]FIG. 14 is a sectional view similar to FIG. 3, but according to athird embodiment of the present invention;

[0044] FIGS. 15 to 17 show a fourth embodiment of the present invention,wherein

[0045]FIG. 15 is a side view of a four-wheel buggy to which the presentinvention is applied;

[0046]FIG. 16 is a plan view of the four-wheel buggy, taken verticallythrough the power unit; and

[0047]FIG. 17 is an enlarged vertical sectional view of a transmittingsystem for the power unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] A first embodiment of the present invention will first bedescribed with reference to FIGS. 1 to 12.

[0049] Referring to FIG. 1, a saddle Sm is mounted on a motorcycle Vm atan upper portion of a body frame Fm supporting a front wheel Wf and arear wheel Wr, and a power unit P is mounted at a lower portion of thebody frame Sm. A fuel tank Tfm is disposed

[0050] As shown in FIGS. 1 and 2, the power unit P is comprised of anengine E and a multi-stage transmission M which are formed integrallyeach other. The engine E includes, as conventionally normal, acrankshaft 2 carried in a crankcase 1 with a pair of left and right ballbearings 3 and 3′ interposed therebetween, and a piston 7 slidablyreceived in a cylinder bore 5 a in a cylinder block 5 and connected tothe crankshaft 2 through a connecting rod 6. The engine E is disposedwith the crankshaft 2 turned in a lateral direction of the motorcycleVm. A cylinder head 4 is coupled to the cylinder block 5 to define acombustion chamber 4 a between the cylinder head 4 and a top surface ofthe piston 7. Provided in the cylinder head 4 are intake and exhaustvalves (not shown) for opening and closing intake and exhaust boresconnected to the combustion chamber 4 a, and a cam shaft 9 adapted toopen and close the intake and exhaust valves. The cam shaft 9 isrotatably carried in the cylinder head 4 in parallel to the crankshaft2.

[0051] A transmission case 8 is integrally connected to the crankcase 1,and input and output shafts 10 and 11 of the multi-stage transmission M,which are disposed in parallel to the crankshaft 2, are supported byopposite left and right sidewalls of the transmission case 8 with ballbearings 12 and 12′; 13 and 13′ interposed therebetween, respectively. Afirst-speed gear train G1, a second-speed gear train G2, a third-speedgear train G3 and a fourth-speed gear train G4 are disposed in the namedorder from the left as viewed in FIG. 2 over the input and output shafts10 and 11. A driven gear G2 b in the second-speed gear train G2 and adriving gear G3 a in the third-speed gear train G3 also serve asshifting gears. When both of the shifting gears G2 b and G3 a are intheir neutral positions, the transmission M is in a neutral state. Whenthe shifting gear G2 b is moved leftwards or rightwards as viewed inFIG. 2, the first-speed gear train GI or the third-speed gear train G3is established. When the shifting gear G3 a is moved leftwards orrightwards as viewed, the second-speed gear train G2 or the fourth-speedgear train G4 is established. The shifting gears G2 b and G3 a areoperated by a known pedal-type changing device or another manualchanging device which is not shown.

[0052] A right end of the crankshaft 2 and a right end of the inputshaft 10 of the transmission M are connected to each other through ashifting clutch Cc, a torque converter T and a primary reducing device14 which are connected together in series outside the crankcase 1 andthe transmission case 8. In this case, especially, the shifting clutchCc, the torque converter T and a driving gear 14 a of the primaryreducing device 14 are mounted on the crankshaft 2 in the order of thedriving gear 14 a, the torque converter T and the shifting clutch Ccfrom the right sidewall of the crankcase 1 toward the outside. A rightside cover 15 a, which covers the shifting clutch Cc, the torqueconverter T and the driving gear 14 a, is bonded to right end faces ofthe crankcase 1 and the trasmission case 8.

[0053] A rotor 17 of a generator 16 is secured to a left end of thecrankshaft 2, and a stator 18 of the generator 16 is mounted to a leftside cover 15 b which is bonded to a left end face of the crankshaft 1to cover the generator 16. A continuous timing transmitting chamber 90is defined in those left sidewalls of the crankcase 1 and the cylinderblock 5, which are opposite to the torque converter T and the primaryreducing device 14. A timing transmitting device 91 for transmitting therotation of the crankshaft 2 through a reduction to one half to the camshaft 9 is accommodated in the transmitting chamber 90. Thus, a group ofthe primary reducing device 14, the torque converter T and the shiftingclutch Cc and a group of the timing transmitting device 91 and thegenerator 16 are disposed at opposite ends of the crankshaft 2 in such amanner that the inside of the crankcase 1, i.e., a crank chamber, issandwiched therebetween.

[0054] As shown in FIGS. 2 and 3, provided in the crankshaft 2 are anupstream supply oil passage 27 a which opens into a right end face ofthe crankshaft 2, a downstream supply oil passage 27 b whichcommunicates with a needle bearing 49 on an outer peripheral surface ofa crank pin supporting a larger end of the connecting rod 6, an orifice48 which communicates directly with both the oil passages 27 a and 27 b,a first flow-in bore 43 a extending radially from the upstream supplyoil passage 27 a toward the shifting clutch Cc, a second flow-in bore 43b extending radially from the upstream supply oil passage 27 a towardthe torque converter T, and a flow-out bore 45 extending radially fromthe downstream supply oil passage 27 b toward the torque converter T. Anoil pumped from an oil reservoir 46 by an oil pump 44 driven by theengine E is fed under a pressure through an oil passage 27 defined inthe right side cover 15 a to the upstream supply oil passage 27 a. Theoil reservoir 46 is defined in bottoms of the crankcase 1, thetransmission case 8 and the right side cover 15 a.

[0055] A chain-type final reducing device 19 which drives the rear wheel(not shown) of the motorcycle is connected to a left end of the outputshaft 11 of the transmission M outside the transmission case 8.

[0056] Referring to FIGS. 2 and 3, the shifting clutch Cc includes acylindrical clutch casing 20 having an end wall 20 a at its one end anda boss 20 b spline-coupled to the crankshaft 2 at its central portion, apressing plate 21 disposed within the clutch casing 20 and slidablyspline-coupled to an outer periphery of the boss 20 b, a pressurereceiving plate 22 oil-tightly secured to an opened end of the clutchcasing 20, and an annular friction clutch plate 23 interposed betweenthe pressing plate 21 and the pressure receiving plate 22. Atransmitting plate 24 of a pump impeller 50, which will be describedhereinafter, is spline-engaged with an inner periphery of the frictionclutch plate 23 (see FIG. 4).

[0057] The pressing plate 21 defines a hydraulic pressure chamber 25between the pressing plate 21 and an end wall 20 a and a peripheral wallof the clutch casing 20. The hydraulic pressure chamber 25 is connectedto the first flow-in bore 43 a in the crankshaft 2 through an inletvalve 26 provided on the boss 20 b of the clutch casing 20, and opens tothe outside of the clutch casing 20 through an outlet valve 28 providedon an outer periphery of the end wall 20 a.

[0058] As shown in FIGS. 3 and 4, provided in the boss 20 b are aplurality of (three in the illustrated embodiment) valve bores 29extending in parallel to the crankshaft 2, and a plurality ofthrough-bores 30 each extending via each of the valve bores 29 throughthe first flow-in bore 43 a to the hydraulic pressure chamber 25. Theinlet valve 26 comprising a spool valve is slidably received in each ofthe valve bore 29. When the inlet valves 26 occupy their right positionsas viewed in FIG. 3 (upper half as viewed in FIG. 3), the through-bores30 are opened, and when the inlet valves 26 occupy their left positions(lower half as viewed in FIG. 3), the through-bores 30 are closed. Toensure the communication between the through-bores 30 in the boss 20 band the first flow-in bore 43 a in the crankshaft 2, it is effective tocut off some of teeth in the coupled spline portions of the crankshaft 2and the boss 20 b.

[0059] A plurality of (three in the illustrated embodiment) outlet bores32 are provided in an outer periphery of the end wall 20 a of the clutchcasing 20 at equal distances in a circumferential direction, and theoutlet valve 28 comprising a reed valve is coupled at its one end bycaulking to the end wall 20 a and capable of opening and closing each ofthe outlet bores 32 on the side of the hydraulic pressure chamber 25.

[0060] Further, guide collars 33 are secured to the end wall 20 a andcommunicates with the outlet bores 32, and a valve opening rod 31 isslidably received in each of the guide collars 33. The valve opening rod31 has an axially extending groove 31 a around an outer peripherythereof. When the valve opening rod 31 occupies a right position asviewed in FIG. 3 (see the upper half as viewed in FIG. 3, and see FIG.7), the closing of the outlet bore 32 by a resilient force of the outletvalve 28 is permitted. When the valve opening rod 31 occupies a leftposition as viewed in FIG. 3 (see the lower half as viewed in FIG. 3,and see FIG. 8), the outlet valve 28 is flexed inwards of the hydraulicpressure chamber 25 to open the outlet bore 32.

[0061] A common valve operating plate 34 is connected to outer ends ofthe inlet valves 26 and the valve opening rods 31. The valve operatingrod 34 is carried on the boss 20 b of the clutch casing 20 for slidingmovement in a lateral direction as viewed in FIG. 3. A stopper ring 35for defining the right position of the valve operating plate 34 islocked to the boss 20 b, and a return spring 36 for biasing the valveoperating plate 34 toward the stopper ring 35 is mounted undercompression between the clutch casing 20 and the valve operating plate34.

[0062] An urging ring 38 is mounted on the valve operating plate 34 witha release bearing 37 interposed therebetween and concentricallysurrounding the boss 20 b, and an arm 39 a fixedly mounted on a shiftingclutch operating shaft 39 is engaged with an outer end face of theurging ring 38. Thus, the valve operating plate 34 can be movedleftwards and rightwards along with the inlet valves 26 and the valveoperating rods 31 in cooperation with the return spring 36 byreciprocally turning the shifting clutch operating shaft 39.

[0063] An electric or electromagnetic shifting clutch actuator 40 isconnected to the shifting clutch operating shaft 39 for turning theshifting clutch operating shaft 39, as shown in FIG. 6. The shiftingclutch actuator 40 receives output signals from an idling sensor 41 fordetecting an idling state of the engine E and a shifting sensor 42 fordetecting the shifting operation of the transmission M, and moves inresponse to these signals to turn the shifting clutch operating shaft 39in a direction to move the valve operating plate 34 leftwards as viewedin FIG. 3.

[0064] The operation of the shifting clutch Cc will be described below.When the engine E is in operation and the idling sensor 41 and theshifting sensor 42 transmit no output signals, the shifting clutchactuator 40 is retained in an inoperative state and hence, the valveoperating plate 34 is retained in its retracted position, i.e., in theright position as viewed in FIG. 3 by a biasing force of the returnspring 36, thereby opening the inlet valves 26 and permitting theclosing of the outlet valves 28. Therefore, the oil pumped from the oilpump 44 is supplied from the upstream supply oil passage 27 a via thefirst inlet bore 43 a and the through bores 30 to the hydraulic pressurechamber 25 in the clutch casing 20 to fill the hydraulic pressurechamber 25.

[0065] The clutch casing 20 is rotated along with the crankshaft 2 andhence, the oil in the hydraulic pressure chamber 25 in the clutch casing20 receives a centrifugal force to generate a hydraulic pressure, andthe pressing plate 21 urges the friction clutch plate 23 against thepressure receiving plate 22 by such hydraulic pressure, whereby thepressing plate 21, the pressure receiving plate 22 and the frictionclutch plate 23 are brought into friction engagement with one another.Namely, the shifting clutch Cc assumes an ON-state to transmit a torqueout from the crankshaft 2 through the friction clutch plate 23 to thetorque converter T.

[0066] On the other hand, during an idling of the engine E or during ashifting operation of the transmission M, the idling sensor 41 or theshifting sensor 42 outputs the output signal, and hence, the shiftingclutch actuator 40 receiving the output signal is operated immediatelyto turn the shifting clutch operating shaft 39 to move the valveoperating plate 34 to the left position as viewed in FIG. 3. This closesthe inlet valves 26 and at the same time, opens the outlet valves 28, asshown in the lower half of FIG. 3. As a result, the supplying of the oilfrom the upstream supply oil passage 27 a to the hydraulic pressurechamber 25 is cut off, and the oil in the hydraulic pressure chamber 25is passed through the outlet bores 32 and the grooves 31 a in the valveoperating rods 31 and discharged to the outside of the clutch casing 20to drop the hydraulic pressure in the hydraulic pressure chamber 25 andto remarkably decrease the urging force of the pressing plate 21 to thefriction clutch plate 23. Therefore, the friction engagement of thethree plates: the pressing plate 21, the pressure receiving plate 22 andthe friction clutch plate 23 is released. Namely, the shifting clutch Ccassumes an OFF state to cut off the transmitting of the torque from thecrankshaft 2 to the torque converter T. The oil discharged to theoutside of the clutch casing 20 is returned to the oil reservoir 46.

[0067] When the rotation of the engine E is accelerated to start thevehicle from such state, or the shifting operation is completed, therebystopping of the output signals of both the idling sensor 41 and theshifting sensor 42, the shifting clutch actuator 40 is immediatelyreturned to its inoperative state, and the valve operating plate 34 isretreated at a stretch to the right position by the biasing force of thereturn spring 36, thereby again opening the inlet valves 26 and at thesame time, closing the outlet valves 28. Therefore, as can be seen fromthe above-described operation, the shifting clutch Cc is restored fromthe OFF state to the ON state without via a half-clutched state or aclutch-slipping state. Namely, the shifting clutch Cc is of an ON andOFF type having no half-clutched area and has a torque capacity which isset larger than that of the torque converter T.

[0068] Referring again to FIG. 3, the torque converter T comprises apump impeller 50, a turbine impeller 51 and a stator impeller 52. Thepump impeller 50 is disposed adjacent the pressure receiving plate 22,and has a boss 50 a which is carried on the crankshaft 2 with a needlebearing 53 interposed therebetween. The transmitting plate 24spline-engaged with the inner periphery of the friction clutch plate 23is secured to an outer surface of the pump impeller 50. Therefore, atransmitted torque from the friction clutch plate 23 is transmittedthrough the transmitting plate 24 to the pump impeller 50.

[0069] A stator shaft 60 is disposed between the boss 50 a of the pumpimpeller 50 and the ball bearing 3′ carrying the crankshaft 2, and iscarried at its right end on the crankshaft 2 with a needle bearing 54interposed therebetween. A boss 52 a of the stator impeller 52 isconnected to the stator shaft 60 by concavo-convex engagement. A statorarm 56 is secured to a left end of the stator shaft 60, with an outerperipheral surface of a cylindrical portion 56 a possessed at anintermediate portion by the stator arm plate 56 being carried on thecrankcase 1 with a ball bearing 57 interposed therebetween. An outerperiphery of the stator arm plate 56 is also carried on the crankcase 1with free wheel 58 interposed therebetween.

[0070] The turbine impeller 51 opposed to the pump impeller 50 has aturbine shaft 59 integrally provided at its center portion, and carriedat its right end on the stator shaft 60 with a needle bearing 61interposed therebetween. The turbine shaft 59 is carried at its left endon an inner peripheral surface of the cylindrical portion 56 a of thestator arm plate 56 with a ball bearing 62 interposed therebetween. Aone-way clutch 64 is provided between the turbine shaft 59 and thecrankshaft 2 to extend through a lateral bore 63 in the stator shaft 60.When a reverse load is applied to the turbine shaft 59, the one-wayclutch 64 is brought into an ON state to directly connect the turbineshaft 59 and the crankshaft 2 to each other.

[0071] As shown in FIG. 3, a clearance defined between the boss 50 a ofthe pump impeller 50, the turbine shaft 59 and the boss 52 a of thestator impeller 52 serves as a fluid inlet 47 i in the torque converterT, and a fluid outlet 47 o in the torque converter T is provided at thatportion of the turbine shaft 59 which extends out of the turbineimpeller 51. The fluid inlet 47 i is communicates with the secondflow-in bore 43 b in the crankshaft 2, and the fluid outlet 47 ocommunicates with the flow-out bore 45 in the crankshaft 2 through thelateral bore 63 in the stator shaft 60. Therefore, when the oil suppliedfrom the oil pump 44 to the upstream supply oil passage 27 a in thecrankshaft 2 enters the second flow-in bore 43 b, it flows through thefluid inlet 47 into an oil chamber defined between the pump impeller 50and the turbine impeller 51 to fill the oil chamber and a hydraulicpressure chamber 77 in a lock-up clutch Lc which will be describedhereinafter, and then flows through the fluid outlet 47 o via theflow-out bore 45 toward the downstream supply oil passage 27 b in thecrankshaft 2.

[0072] The driving gear 14 a of the primary reducing device 14 isintegrally formed on the turbine shaft 59, and the driven gear 14 bmeshed with the driving gear 14 a is spline-coupled to the input shaft10 of the transmission M. The primary reducing device 14 constructed inthe above manner is disposed between the crankcase 1 and the torqueconverter T.

[0073] The operation of the torque converter T will be described below.

[0074] When the output torque from the crankshaft 2 is transmittedthrough the shifting clutch Cc which is in the ON state to the pumpimpeller 50, it is transmitted fluidally to the turbine impeller 51 bythe action of the oil filling the inside of the torque converter T. If atorque amplifying effect has been generated between both the impellers50 and 51 at this time, a reaction force attendant thereon is borne bythe stator impeller 52, and the stator impeller 52 is fixedly supportedon the crankcase 1 by the locking action of the free wheel 58. If notorque amplifying effect is generated, the stator impeller 52 can beraced by a racing action of the free wheel 58 and hence, the threeimpellers: the pump impeller 50, the turbine impeller 51 and the statorimpeller 52 are all rotated in the same direction.

[0075] The torque transmitted from the pump impeller 50 to the turbineimpeller 51 is transmitted through the primary reducing device 14 to theinput shaft 10 of the transmission M and then transmitted sequentiallyvia the established shifting gear trains G1 to G4, the output shaft 11and the finally reducing device 19 to the rear wheel (not shown) todrive the rear wheel.

[0076] During conduction of an engine brake during traveling of thevehicle, the one-way clutch 64 is brought into the ON state byapplication of the reverse load torque to the turbine shaft 59.Therefore, the turbine shaft 59 and the crankshaft 2 are connecteddirectly to each other, whereby the reverse load torque is transmittedto the crankshaft 2 without via the torque converter T. Thus, it ispossible to provide a good engine brake effect.

[0077] Referring again to FIG. 3, a lock-up clutch Lc is providedbetween the pump impeller 50 and the turbine impeller 51 and capable ofdirectly connecting the pump impeller 50 and the turbine impeller 51 toeach other. The lock-up clutch Lc includes a cylindrical pump extension70 which is connected to the outer periphery of the pump impeller 50 tosurround the turbine impeller 51, a pressing plate 72 which is slidablyspline-fitted over a support tube 71 rotatably carried on the outerperipheral surface of the turbine shaft 59, a pressure receiving plate73 which is oil-tightly secured to an end of the pump extension 70 in anopposed relation to the pressing plate 72 and which is spline-fittedover the support tube 71, and a annular friction clutch plate 74interposed between the pressing plate 72 and the pressure receivingplate 73. The friction clutch plate 74 has an outer peripheryspline-engaged with a transmitting plate 75 secured to the outer surfaceof the turbine impeller 51 (see FIG. 9). The retreated position of thepressing plate 72 to the pressure receiving plate 73 is defined by astopper ring 76 locked to the support tube 71.

[0078] A hydraulic pressure chamber 77 is defined in the inside of thepump extension 70 by the pressure receiving plate 73, and communicateswith the insides of the pump impeller 50 and the turbine impeller 51through opposed clearances between the pump impeller 50 and the turbineimpeller 51. When the oil is filled in the hydraulic pressure chamber77, during the operation of the torque converter T, the hydraulicpressure chamber 77 is at a high pressure, as are the insides of thepump impeller 50 and the turbine impeller 51.

[0079] As shown in FIGS. 3, 11 and 12, a plurality of (three in theillustrated embodiment) valve bores 78, 79 are provided in each of thepressing plate 72 and the pressure receiving plate 73 atcircumferentially equal distances on the side of the inner periphery ofthe friction clutch plate 74, and a control valve 80 comprising a reedvalve capable of opening and closing the valve bores 78 in the pressingplate 72 on the side of the hydraulic pressure chamber 77 is coupled atits one end to the pressing plate 72 by caulking.

[0080] The valve bores 78 and 79 in the pressing plate 72 and thepressure receiving plate 73 are disposed coaxially with each other, anda control rod 81 for controlling the opening and closing of the controlvalve 80 is slidably received in the valve bores 78 and 79. The controlrod 81 has an axially extending communication groove 81 a in its outerperiphery. When the control rod 81 occupies a left position as viewed inFIG. 3 (see upper half of FIG. 3 and see FIG. 11), the closing of thevalve bore 78 by the resilient force of the control valve 80 ispermitted, and at the same time, the inner periphery of the frictionclutch plate 74 is opened to the outside of the valve bore 79 in thepressure receiving plate 73 by the communication groove 81 a in thecontrol rod 81. When the control rod 81 occupies a right position asviewed in FIG. 3 (see lower half of FIG. 3 and see FIG. 12), the valvebore 79 in the pressure receiving plate 73 is closed by the control rod81, and at the same time, the control valve 80 is flexed inwards of thehydraulic pressure chamber 77, thereby permitting opposite side faces ofthe pressing plate 72 to communicate with each other through thecommunication groove 81 a on the side of the inner periphery of thefriction clutch plate 74.

[0081] A valve operating plate 82 is connected to an outer end of thecontrol rod 81. The valve operating plate 82 is carried on the supporttube 71 for sliding movement in a lateral direction as viewed in FIG. 3.A stopper ring 83 for defining a left position of the valve operatingplate 82 is locked to the support tube 71, and a return spring 84 forbiasing the valve operating plate 82 toward the stopper ring 83 ismounted under compression between the pressure receiving plate 73 andthe valve operating plate 82.

[0082] An arm 86 a of a lock-up clutch operating shaft 86 (an operatingmeans) is engaged with the valve operating plate 82 through a releasebearing 85 which is disposed concentrically with the support tube 71, sothat the valve operating plate 82 can be moved laterally along with thecontrol rod 81 in cooperation with the return spring 84 by reciprocallyturning the lock-up clutch operating shaft 86.

[0083] An electric or electromagnetic lock-up clutch actuator 87 isconnected to the lock-up clutch operating shaft 86 for turning thelock-up clutch operating shaft 86, as shown in FIG. 6. The lock-upclutch actuator 87 receives an output signal from a vehicle speed sensor88 for detecting a vehicle speed equal to or lower than a predeterminedvalue, and moves in response to the signal to turn the lock-up clutchoperating shaft 86 in a direction to move the valve operating plate 82rightwards as viewed in FIG. 3.

[0084] The operation of the lock-up clutch Lc will be described below.When the vehicle speed sensor 38 detects a vehicle speed equal to orlower than the predetermined value to deliver an output signal, thelock-up clutch actuator 87 is operated under reception of the signal toturn the lock-up clutch operating shaft 86, thereby moving the valveoperating plate 82 rightwards as viewed in FIG. 3. With this movement,the control rod 81 opens the control valve 80 to permit the oppositeside faces of the pressing plate 72 to communicate with each otherthrough the communication groove 81 a, as shown in the lower half of theFIG. 3 and in FIG. 2. Therefore, the hydraulic pressure in the hydraulicpressure chamber 77 is applied equally to the opposite side faces of thepressing plate 72, and the pressing plate 72 is urged to the retreatedposition by the urging force of the control rod 81 to the control valve80, whereby the friction engagement of the three plates: the pressingplate 72, the pressure receiving plate 73 and the friction clutch plate74 does not occur, and the lock-up clutch Lc assumes the OFF state.Therefore, in this state, the relative rotation of the pump impeller 50and the turbine impeller 51 is possible and hence, a torque amplifyingeffect can be provided. In this case, the valve bores 79 in the pressurereceiving plate 73 are closed by the control rods 81 and hence, theuseless leakage of the hydraulic pressure from the hydraulic pressurechamber 77 to the valve bores 79 can be prevented.

[0085] When the vehicle speed is increased up to a level equal to orhigher than the predetermined value, and the vehicle speed sensor 88stops the delivery of the output signal, the lock-up clutch actuator 87returns to the inoperative state, and the valve operating plate 82 isretreated to the left position by the biasing force of the return spring84, as shown in the upper half of FIG. 3 and in FIG. 11, therebypermitting the closing of the valve bores 78 by the control valve 80,and opening the inner periphery of the friction clutch plate 74 to theoutside of the valve bores 79 through the communication grooves 81 a inthe control rods. Therefore, the pressing plate 72 receives thehydraulic pressure in the hydraulic pressure chamber 77 on its innersurface to urge the friction clutch plate 74 against the pressurereceiving plate 73. As a result, the pressing plate 72, the pressurereceiving plate 73 and the friction clutch plate 74 are brought intoengagement with one another, whereby the lock-up clutch Lc is broughtinto the ON state to connect the pump impeller 50 and the turbineimpeller 51 directly to each other. Therefore, during traveling of themotorcycle Vm at a high speed, the slipping of both the impellers 50 and51 can be eliminated to enhance the transmitting efficiency.

[0086] During operation of the engine E, the oil discharged from the oilpump 44 flows first into the upstream supply oil passage 27 a and thenvia the first flow-in bore 43 a into the hydraulic pressure chamber 25in the shifting clutch Cc to contribute the operation and cooling of theshifting clutch Cc. In addition, the oil flows via the second flow-inbore 43 b into the oil chamber defined between the pump impeller 50 andthe turbine impeller 51 and into the hydraulic pressure chamber 77 inthe lock-up clutch Lc to contribute to the operation and cooling of thetorque converter T and the lock-up clutch Lc. The oil discharged fromthe hydraulic pressure chamber 77 through the flow-out bore 45 into thedownstream supply oil passage 27 b is supplied to the needle bearing 49around the outer periphery of the crank pin to contribute to thelubrication of the needle bearing 49. The oil finishing the lubricationis scattered the surroundings with the rotation of the crankshaft 2 tolubricate the piston 7 and the like. The oil pump 44 originally acts tosupply the lubricating oil to the engine E, but the oil is utilized asan operating oil for the shifting clutch Cc, the torque converter T andthe lock-up clutch Lc. Therefore, it is unnecessary to mount anexclusive oil pump for supplying the operating oil, thereby enabling thesimplification of the arrangement.

[0087] The upstream and downstream supply oil passages 27 a and 27 bprovided in the crankshaft 2 communicates directly with each otherthrough the orifice 48 and hence, a portion of the oil fed from the oilpump 44 to the upstream supply oil passage 27 a passes through theorifice 48 to the downstream supply oil passage 27 b without via thetorque converter T and the like. Therefore, the proportion ofdistribution of the oil to the torque converter T and the engine E canbe determined freely by selection of the orifice 48.

[0088] On the other hand, in the torque converter T, the somewhattransmission of the torque occurs between the pump impeller 50 and theturbine impeller 51 even during idling of the engine E. However, theshifting clutch Cc is controlled to the OFF state during idling of theengine E and hence, even if the first-speed gear train G1 of themulti-stage transmission M has been established, the transmission of apower to the shifting clutch Cc and the like can be cut off,irrespective of the presence of the torque converter T, therebypreventing a creeping phenomenon. This means that the transmittingmembers of the multi-stage transmission M are put in an unloaded state.Therefore, even when the shifting gear G2 b is shifted leftwards asviewed in FIG. 2 to establish the first-speed gear train G1 for startingthe motorcycle Vm, this shifting can be conducted smoothly without beingaccompanied by a torque shock. When the rotation of the engine E isaccelerated to start the motorcycle, the shifting clutch Cc is broughtat a stretch to the ON state beyond the half-clutched area, but theaccompanying torque shock is absorbed by the action of mutual slippingof the pump impeller 50 and the turbine impeller 51 of the torqueconverter T, whereby the smooth starting of the motorcycle can becarried out with the aid of the amplifying effect. This can contributeto an improvement in riding comfort.

[0089] Even when the shifting gears G2 b and G3 a are shifted in adesired direction during traveling of the motorcycle to conduct adesired shifting, the shifting clutch Cc is controlled each time to theOFF state, as described above, and the transmitting members of themulti-stage transmission M are brought into their unloaded states.Therefore, the shifting can be conducted smoothly without beingaccompanied by a torque shock. Even after the shifting, the shiftingclutch Cc is brought at a stretch to the ON state beyond thehalf-clutched area, but the accompanying torque shock is absorbed by theaction of mutual slipping of the pump impeller 50 and the turbineimpeller 51 of the torque converter T. Therefore, a sense ofincompatibility is not provided to an occupant, and an improvement inriding comfort is provided.

[0090] In this way, the torque shock produced with turning-on and off ofthe shifting clutch is absorbed to the torque converter T and hence, theshifting clutch Cc can be constructed into an on and off type having nohalf-clutched area. In addition, it is possible to avoid the heating andwearing of the friction portion due to the half-clutching to enhance thedurability of the shifting clutch Cc.

[0091] The torque capacity of the shifting clutch Cc is set larger thanthat of the torque converter, as described above, and hence, even in afully loaded state, the slipping of the shifting clutch Cc can beprevented, and the durability of the shifting clutch can be ensured.

[0092] In addition, the crankshaft 2 is rotated at a high speed by theinput shaft 10 of the multi-stage transmission M driven through thereducing device 14 by the crankshaft 2. Therefore, the transmittedtorque borne by the torque converter T and the shifting clutch Ccmounted to the crankshaft 2 is relatively small and hence, thecapacities of the torque converter T and the shifting clutch Cc can bereduced correspondingly, leading to the compactness of the torqueconverter T and the shifting clutch Cc. In addition, the compactness ofthe power unit P can be provided despite the provision of both thetorque converter T and the shifting clutch Cc.

[0093] Moreover, among the primary reducing device 14, the torqueconverter T and the shifting clutch Cc, the primary reducing device 14is disposed nearest to the right sidewall of the crankcase 1, and thetorque converter T is disposed nearer to the right sidewall. Therefore,the flexing moment applied to the crankshaft 2 and the input shaft 10with the operation of the primary reducing device 14 can be minimized.In addition, the weight of the torque converter T is larger than that ofthe shifting clutch Cc, but the flexing moment applied to the crankshaft2 due to the weights of the torque converter T and the shifting clutchCc can be minimized, whereby the durability of the crankshaft 2, theinput shaft 10 and the bearings 3′ and 12′ supporting the crankshaft 2and the input shaft 10 can be enhanced in cooperation with thecompactness of the torque converter T and the shifting clutch Cc.

[0094] Additionally, since the group of the primary reducing device 14,the torque converter T and the shifting clutch Cc and the group of thetiming transmitting device 91 and the generator 16 are disposed on thecrankshaft 2 on the opposite sides with the crank chamber interposedtherebetween, as described above, the lateral distribution of the weightof the power unit P can be equalized. Moreover, even in a 4-cycleengine, the primary reducing device 14 can be disposed nearest to theright sidewall of the crankcase 1 without being interfered in any way bythe timing transmitting device 91, and the durability of the crankshaft2, the input shaft 10 and the bearings 3′ and 12′ supporting thecrankshaft 2 and the input shaft 10 can be ensured.

[0095] Further, since the generator 16 and the torque converter T on thecrankshaft 2 are disposed coaxially, the rotational vibration generatedin the generator 16 can be absorbed by the torque converter T tocontribute to the silence of the power unit P.

[0096] A second embodiment of the present invention shown in FIG. 13will now be described.

[0097] The second embodiment is different from the previously describedembodiment in respect of that a lock-up clutch Lc′ is constructed intoan automatically controlled type depending on the rotational speed ofthe pump impeller 50. More specifically, the lock-up clutch Lc′ includesa cylindrical pump extension 70 connected to an outer periphery of thepump impeller 50 and surrounding the turbine impeller 51, a pressurereceiving plate 93 which is rotatably carried on the turbine shaft 59and oil-tightly coupled to an opened end of the pump extension 70, apressing plate 94 which is slidably carried on the turbine shaft 59 anddisposed in an opposed relation to an inner surface of the pressurereceiving plate 93, an annular friction clutch 95 interposed between thepressing plate 94 and the pressure receiving plate 93, a dished orbelleville return spring 96 interposed between the pump extension 70 andthe pressing plate 94 for biasing the pressing plate 94 in a directionopposite to the pressure receiving plate 93. The friction clutch plate95 has an outer periphery engaged with the transmitting plate 75 securedto the outer surface of the turbine impeller 51. The pressure receivingplate 93 and the pressing plate 94 have a dog 97 and a recess 98provided in opposed surfaces thereof, respectively and engaged with eachother, so that the pressure receiving plate 93 and the pressing plate 94can be slid in an axial direction relative to each other, while beingrotated in unison with each other.

[0098] A hydraulic pressure chamber 99 is defined in the inside of thepump extension 70 by the pressure receiving plate 93. The hydraulicpressure chamber 99 communicates with the insides of the pump impeller50 and the turbine impeller 51 through opposed clearances between thepump impeller 50 and the turbine impeller 51, so that the oil is filledin the hydraulic pressure chamber 99.

[0099] Provided in the pressure receiving plate 93 are an escape bore100 which opens the inner periphery of the friction clutch plate 95 tothe outside of the pressure receiving plate 93, and an air-vent groove101 extending axially in an inner peripheral surface of the pressurereceiving plate 93.

[0100] Another arrangement is the same as in the arrangement in thefirst embodiment and hence, portions or components corresponding tothose in the first embodiment are designated by like referencecharacters and the description of them is omitted.

[0101] When the rotational speed of the pump impeller 50 is equal to orlower than a predetermined value, the centrifugal force of the oilfilling the hydraulic pressure chamber 99 within the pump extension 70is small. For this reason, the hydraulic pressure in the hydraulicpressure chamber 99 does not rise, and the pressing plate 94 has beenreturned to its retreated position by the biasing force of the returnspring 96 to release the friction clutch plate 95. Therefore, thelock-up clutch Lc′ is in its OFF state.

[0102] During this time, the oil in the hydraulic pressure chamber 99flows out to the outside through the escape bore 100 in the pressurereceiving plate 93, but the amount thereof is extremely small.Therefore, the flowing-out of the oil does not hinder the rising of thehydraulic pressure in the hydraulic pressure chamber 99.

[0103] When the rotational speed of the pump impeller 50 exceeds thepredetermined value, the centrifugal force of the oil in the hydraulicpressure chamber 99 increases correspondingly to rise the hydraulicpressure in the hydraulic pressure chamber 99. Therefore, the pressingplate 94 is advanced toward the pressure receiving plate 93 by suchrisen hydraulic pressure to clamp the friction clutch plate 95 betweenthe pressing plate 94 and the pressure receiving plate 93, whereby thelock-up clutch Lc′ is brought into its ON state. The lock-up clutch Lc′in the ON state connects the pump impeller 50 and the turbine impeller51 directly to each other and hence, the mutual slipping of both theimpellers 50 and 51 can be eliminated to enhance the transmittingefficiency.

[0104] In this case, the rising of the hydraulic pressure does not occuron the side of the inner periphery of the friction clutch plate 95,because the oil flows out through the escape bore 100. Therefore, alarge difference in pressure is produced between the opposite surfacesof the pressure plate 94, whereby the clamping of the friction clutchplate 95 is carried out effectively.

[0105] Thus, by utilizing the centrifugal hydraulic pressure in thehydraulic pressure chamber 99 within the pump extension 70 connected tothe pump impeller 50, it can be achieved easily that the automaticcontrolling of the lock-up clutch Lc′ depends on the rotational speed ofthe pump impeller 50.

[0106] A third embodiment of the present invention shown in FIG. 14 willbe described below.

[0107] The third embodiment is different from the second embodiment inrespect of that a lock-up clutch Lc″ is constructed into anautomatically controlled type depending on the rotational speed of theturbine impeller 52. The lock-up clutch Lc″ is disposed outside a torqueconverter side-cover 105 which is oil-tightly coupled to the pumpextension 70 of the pump impeller 50 to cover the turbine impeller 51.The torque converter side-cover 105 is rotatably carried on an outerperiphery of the turbine shaft 59, and the inside thereof communicateswith an oil chamber defined between the pump impeller 50 and the turbineimpeller 51, and is filled with a working oil, as is the oil chamber.

[0108] The lock-up clutch Lc″ includes a flat clutch cylinder 106 aspline-coupled to a left end of the turbine shaft 59 with its opened endturned toward the torque converter side-cover 105, a pressing piston 107slidably received in a cylinder bore 106 in the clutch cylinder 106 witha seal member 113 interposed therebetween to define a hydraulic pressurechamber 108 between the pressing piston 107 and an end wall of theclutch cylinder 106, a pressure receiving ring 109 locked to an innerperipheral surface of the clutch cylinder 106 at a location closer tothe opened end, a plurality of (two in the illustrated embodiment)annular driven friction clutch plates 111, 111 which are slidablyspline-engaged with the inner peripheral surface of the clutch cylinder106 between the pressure receiving ring 109 and the pressing piston 107,an annular driving friction clutch plate 110 which is interposed betweenthe driven friction clutch plates 111, 111 and which has an innerperipheral surface axially slidably engaged with a plurality oftransmitting claws 112 projectingly provided on an outer surface of thetorque converter side-cover 105, and a piston return spring 114 disposedbetween the pressing piston 107 and the torque converter side-cover 105on the side of the inner peripheries of the driving and driven frictionclutch plates 110 and 111 for biasing the pressing piston 107 toward thehydraulic pressure chamber 108. The clutch cylinder 106 and the pressingpiston 107 have dogs 115 and recesses 116 provided in opposed surfacesthereof, respectively and engaged with each other, so that the clutchcylinder 106 and the pressing piston 107 can be slid in an axialdirection relative to each other, while being rotated in unison witheach other.

[0109] A fluid outlet 47 o and an inlet bore 117 are provided in theturbine shaft 59, and permit the inside of the torque converterside-cover 105 and the hydraulic pressure chamber 108 in the clutchcylinder 106 to communicate with the inner periphery of the turbineshaft 59. Thus, the inside of the torque converter side-cover 105 andthe hydraulic pressure chamber 108 in the clutch cylinder 106 are putinto communication with each other through the fluid outlet 47 o and theinlet bore 117 and through the inside of the turbine shaft 59.

[0110] A plurality of escape bores 118 are provided in a peripheral wallof the clutch cylinder 106 at circumferentially equal distances to openthe hydraulic pressure chamber 108 to the outside of the clutch cylinder106. An annular groove 119 is provided in the inner peripheral surfaceof the clutch cylinder 106 to permit the communication between theescape bores 118, and a centrifugal valve 120 is disposed in the annulargroove 119 and closes the escape bores 118 by a centrifugal force, whenthe rotational speed of the clutch cylinder 106 is equal to or higherthan a predetermined value. The centrifugal valve 120 is comprised of afree-end ring made of a single resilient wire material, with at leastone end 120 a thereof engaged in one of the recesses 116 in the pressingpiston 107, so that the centrifugal valve 120 is rotated along with thepressing piston 107 and thus the clutch cylinder 106. The centrifugalvalve 120 is designed, so that it is contracted radially to open theescape bores 118 in its free state, but when the rotational speed of theclutch cylinder 106 is equal to or higher than predetermined value, thecentrifugal valve 120 is expanded radially by the centrifugal force tocome into close contact with a bottom surface of the annular groove 119to close all the escape bores 118.

[0111] Another arrangement is the same as the arrangement in the firstembodiment and hence, portions and components corresponding to those inthe first embodiment are designated by like reference characters and thedescription of them is omitted.

[0112] When the oil supplied from the oil pump 44 to the upstream supplyoil passage 27 a in the crankshaft 2 enters the second flow-in bore 43b, the oil flows into the oil chamber between the pump impeller 50 andthe turbine impeller 51 through the fluid inlet 47 i to fill the oilchamber and the inside of the torque converter side-cover 105, and thenflows through the fluid outlet 47 o into the turbine shaft 59. The oilflowing out of the turbine shaft 59 is diverted into the inlet bore 117and the flow-out bore 45. The oil entering the inlet bore 117 flows intothe hydraulic pressure chamber 108 in the lock-up clutch Lc′, while theoil entering the flow-out bore 45 flows to the downstream supply oilpassage 27 b in the crankshaft 2, as in the previous embodiment.

[0113] The clutch cylinder 106 of the lock-up clutch Lc″ isspline-coupled to the turbine shaft 59, and it is rotated along with theturbine shaft 59. Therefore, when the rotational speed of the turbineshaft 59 is equal to or lower than the predetermined value, thecentrifugal valve 120 is maintained in its contracted state against thecentrifugal force to open the escape bores 118, so that the oil flowingthrough the inlet bore 117 into the hydraulic pressure chamber 108 flowsout of the clutch cylinder 106 through the escape bores 118. Therefore,the hydraulic pressure in the hydraulic pressure chamber 108 does notrise, whereby the pressing piston 107 is retained in its retreatedposition by the biasing force of the piston return spring 114, and thedriving and driven friction clutch plate 110 and 111 are put into theirnon-engaged states. Namely, the lock-up clutch Lc″ is in the OFF state.

[0114] In this case, if a foreign matter such as a cut powder and a wornpowder exists in the hydraulic pressure chamber 108, the foreign mattercan be discharged out of the clutch cylinder 106 through the escapebores 118 along with the oil.

[0115] When the rotational speed of the turbine shaft 59 exceeds thepredetermined value, the centrifugal valve 120 rotated along with theturbine shaft 59 is expanded by an own increased centrifugal force toclose all the escape bores 118. As a result, the hydraulic pressurechamber 108 is filled with the oil supplied through the inlet bore 117,and a hydraulic pressure is developed in the hydraulic pressure chamber108 by the centrifugal force of the oil. Thus, the pressing piston 107is advanced toward the pressure receiving ring 109 by such developedhydraulic pressure to bring the driving and driven friction clutchplates 110 and 111 into the friction- engaged states, whereby thelock-up clutch Lc″ is brought into the ON state. The lock-up clutch Lc″in the ON state brings the pump impeller 50 and the turbine impeller 59into directly connected states and hence, the mutual slipping of thepump impeller 50 and the turbine impeller 59 can be eliminated toenhance the transmitting efficiency.

[0116] When the rotational speed of the turbine impeller 59 reduces tolower than the predetermined value, the centrifugal valve 120 openesagain and hence, the remaining pressure in the hydraulic pressurechamber 108 can be released promptly through the escape bores 118.Therefore, the turning-off performance of the lock-up clutch Lc″ can beenhanced.

[0117] Thus, by utilizing the centrifugal hydraulic pressure in thehydraulic pressure chamber 108 within the clutch cylinder 106 connectedto the turbine impeller 59, it can be achieved easily that the automaticcontrolling of the lock-up clutch Lc depends on the rotational speed ofthe turbine impeller 59.

[0118] Finally, a fourth embodiment of the present invention shown inFIGS. 15 to 17 will be described below.

[0119] Referring first to FIGS. 5 and 16, in a four-wheel buggy Vb, afuel tank Tfb and a saddle Sb are mounted respectively at a frontlocation and a rear location on an upper portion of a body frame Fbwhich supports a pair of front wheels Wf a and Wfb and a pair of rearwheels Wra and Wrb, and a power unit P is mounted on a lower portion ofthe body frame Fb. Left and right front-wheel driving shafts 121 a and121 b connected to the left and right front wheels Wfa and Wfb,respectively, are connected to each other by a differential 122, and theleft and right rear wheels Wra and Wrb are connected directly to eachother by a single rear-wheel driving shaft 123.

[0120] The power unit P is disposed with a crankshaft 2 of an engine Eturned laterally of the four-wheel buggy Vb. A driving shaft 126 isdisposed longitudinally adjacent a generator 16 of the power unit P andconnected to an output shaft 11 of a transmission M through a bevel geartransmitting device 125. The driving shaft 126 is connected at its frontend to the differential 122 through a front propeller shaft 128 and abevel gear reducing device 129 and at its rear end to the rear-wheeldriving shaft 123 through an adjustable joint 130, a rear propellershaft 131 and a bevel gear reducing device 132. Therefore, the frontwheels Wf a and Wfb and the rear wheels Wra and Wrb can be driven by apower transmitted from the power unit P to the driving shaft 126.

[0121] As shown in FIG. 17, the power unit P in the fourth embodiment isdifferent from that in the first embodiment in respect of thearrangements of a shifting clutch Cc′ and a torque converter T′.

[0122] The shifting clutch Cc′ includes a driving plate 135spline-fitted over the crankshaft 2 and secured thereto by a nut 134,and a bottomed cylindrical clutch outer 137 slidably carried on asupport tube 136 which is integrally and projectingly provided on anouter surface of the driving plate 135. The driving plate 135 isdisposed adjacent an end wall of the clutch outer 137 and has an outerperiphery spline-coupled to an inner periphery of the clutch outer 137.A clutch inner 138 is disposed coaxially within the clutch outer 137,and a plurality of annular driving friction plates 139 slidablyspline-engaged with an inner periphery of a cylindrical portion of theclutch outer 137 and a plurality of annular driven friction plates 140slidably engaged with an outer periphery of the clutch inner 138 aredisposed in an alternately laminated manner. In this case, two drivingfriction plates 139, 139 are disposed inside and outside the group ofthe friction plates 139 and 140, and a pressure receiving ring 141facing an outer surface of the outer driving friction plate 139 islocked to the inner periphery of the cylindrical portion of the clutchouter 137.

[0123] A spacing spring 142 is mounted under compression between boththe driving friction plates 139 and 139 for biasing the driving frictionplates 139 and 139 in a spacing direction. A flange 138 a projectingprovided on the outer periphery of the clutch inner 138 is opposed tothe inner driven friction plate 140.

[0124] A plurality of centrifugal weights 143 are swingably mounted tothe driving plate 135 by a pivot 144, and disposed so that an urging armportion 143 a of each centrifugal weight 143 can urge the inner drivingfriction plate 139. A stopper 145 is mounted on the support tube 136 ofthe driving plate 135 for defining a limit of sliding movement in anoutward direction of the clutch outer 137 (in a rightward direction asviewed in FIG. 17), and a clutch spring 146 is mounted between thedriving plate 135 and the clutch outer 137 for biasing the clutch outer137 toward the stopper 145.

[0125] An annular transmitting member 148 is connected to the clutchinner 138 through a known reverse-load transmitting screw mechanism 147,and spline-coupled to an outer periphery of a boss 50 a of a pumpimpeller 50 of the torque converter T′.

[0126] During idling of the engine E, the rotational speed of thedriving plate 135 rotated along with the crankshaft 2 is low, and thecentrifugal force of the weight portion of the centrifugal weight 143 issmall. Therefore, the urging force of the urging arm portion 143 a tothe driving friction plate 139 is also small. Therefore, the drivingfriction plates 139, 139 on the opposite sides have been spaced apartfrom each other by the biasing force of the spacing spring 142 torelease the driven friction plates 140, and the shifting clutch Cc′ isin its OFF state. Therefore, the shifting clutch Cc′ in the OFF statecuts off the transmission of the power from the crankshaft 2 to the pumpimpeller 50 of the torque converter T′ and hence, even if a wheel brakeis not operated, it is possible to prevent the very slow-speed forwardmovement of the four-wheel buggy Vb due to a creep effect provided bythe torque converter T′.

[0127] When the rotational speed of the engine E is increased to equalto or higher than a predetermined value, the centrifugal force of theweight portion of the centrifugal weight 143 is increased with suchincrease in rotational speed, whereby the urging arm portion 143 astrongly urges the group of the driving and driven friction plates 139and 140 against the pressure receiving ring 141 to bring the driving anddriven friction plates 139 and 140 into friction engagement with eachother. Therefore, the shifting clutch Cc′ is automatically brought intothe ON state to transmit the power of the crankshaft 2 from the clutchinner 138 through the transmitting member 148 to the pump impeller 50 ofthe torque converter T′.

[0128] When the urging force of the centrifugal weights 143 to the groupof the driving and driven friction plates 139 and 140 exceeds a presetload of the clutch spring 146, the clutch outer 137 is displacedleftwards as viewed in FIG. 17 while flexing the clutch spring 146.Moreover, the centrifugal weights 143 are thereafter received by astopper ring 157 on the clutch outer 137, so that the further outwardswinging movement is inhibited. The force of mutual pressure contactbetween the driving and driven friction plates 139 and 140 is notincreased to larger than the load of the clutch spring 146.

[0129] The clutch outer 137 has a boss 137 a protruding on its outersurface, and a release cam 150 is mounted on the boss 137 a with arelease bearing 149 interposed therebetween. A stationary cam 152mounted to the right side-cover 15 a through an adjusting bolt 151 isopposed to the release cam 150, and a ball 153 is mounted on thestationary cam 152 and engaged in a recess 150 a in the release cam 150.

[0130] The release cam 150 includes an arm 154 which has a notch 154 aat its tip end and which protrudes radially, and a tip end of a clutcharm 156 secured to a change spindle 155 used for switching operation ofthe transmission M is engaged in the notch 154 a.

[0131] Thus, when the change spindle 155 is turned for switching of thetransmission M during traveling of the four-wheel buggy Vb, the clutcharm 156 turns the release cam 150 in first half of such turning movementof the change spindle 155, and the release cam 150 pushes the ball 153on the stationary cam 152 out of the recess 150 a with the turningmovement of the release cam 150. A reaction force produced at that timecauses the clutch outer 137 to be urged leftwards as viewed in FIG. 17against the load of the clutch spring 146 through the release bearing149, thereby spacing the pressure receiving ring 141 apart from thegroup of the driving and driven friction plates 139 and 140. On theother hand, the outward swinging movement of the centrifugal weights 143is inhibited by the stopper ring 157, as described above, and the urgingarm portion 143 a is stopped at a previous urging position for thedriving and driven friction plates 139 and 140. Therefore, the drivingand driven friction plates 139 and 140 are reliably spaced apart fromeach other, whereby the shifting clutch Cc′ is brought into the OFFstate.

[0132] The second half of the turning movement of the change spindle 155is placed at the service of switching of the transmission M. After theswitching of the transmission M, the release cam 150 is returned to itsoriginal position with the returning movement of the change spindle 155,and the shifting clutch Cc′ is returned to the ON state by cooperationof the biasing force of the clutch spring 146 with the centrifugal forceof the connected centrifugal weights 143.

[0133] In the torque converter T′, the boss 50 a of the pump impeller 50spline-coupled to the transmitting member 148 is carried on thecrankshaft 2 with a ball bearing 159 interposed therebetween, and theturbine shaft 59 connected to the turbine impeller 51 is carried on thestator shaft 60 with left and right needle bearings 160 and ballbearings 161 with interposed therebetween. The boss 52 a of the statorimpeller 52 is carried on the crankshaft 2 with ball bearings 162 orneedle bearings interposed therebetween, and is spline-coupled to thestator shaft 60.

[0134] A torque converter side-cover 163 is oil-tightly coupled to thepump extension 70 connected to the pump impeller 50 to cover the outsideof the turbine impeller 51, and a one-way clutch 64 is interposedbetween the torque converter side-cover 163 and the turbine shaft 59 fortransmitting only a reverse load torque from the turbine shaft 59 to thetorque converter side-cover 163. Therefore, when a reverse load torqueapplied to the driving shaft 126 is transmitted via the transmission Mand the primary reducing device 14 to the turbine shaft 59 duringconduction of an engine brake, the one-way clutch 64 is brought into aconnected state to transmit the reverse load torque from the pumpextension 70 to the pump impeller 50 and the transmitting member 148.

[0135] When the reverse load torque has been transmitted to thetransmitting member 148, the clutch inner 138 in the shifting clutch Cc′is urged leftwards as viewed in FIG. 17 by operation of the screwmechanism 147, whereby the flange 138 a of the clutch inner 138 urgesthe group of the driving and driven friction plates 139 and 140 againstthe pressure receiving ring 141 with the inner driving friction plate139 left, and hence, the shifting clutch Cc′ is brought into the ONstate. Therefore, the reverse load torque is transmitted to thecrankshaft 2 to provide a good engine brake effect.

[0136] A partition wall 165 is provided on the crankshaft 2 forpartitioning the upstream and downstream supply oil passages 27 a and 27b from each other. A partitioning plug 166 is press-fitted into theupstream supply oil passage 27 a for bisecting the upstream supply oilpassage 27 a into an upstream section and a downstream section.

[0137] In the shifting clutch Cc′, an oil chamber 168 is defined in thesupport tube 136 with an opened surface closed by a lid 167, andcommunicates with the inner periphery of the clutch inner 138 through athrough-bore 169. The oil chamber 168 also communicates with theupstream section and the downstream section of the upstream supply oilpassage 27 a through a flow-in bore 170 and a flow-out bore 171 whichare provided in the crankshaft 2.

[0138] In the torque converter T′, a first small oil chamber 172 isprovided on the right of the boss 52 of the stator impeller 52, and asecond small oil chamber 173 is provided on the left of the boss 52 a.The first small oil chamber 172 communicates with the oil chamberdefined between the pump impeller 50 and the turbine impeller 51 andalso with the downstream section of the upstream supply oil passage 27 athrough a flow-in bore 175 provided in the crankshaft 2. The secondsmall oil chamber 173 communicates with the oil chamber defined betweenthe turbine impeller 51 and the stator impeller 52 and also with thedownstream supply oil passage 27 b through a flow-out bore 176 providedin the crankshaft 2.

[0139] Further, the first and second small oil chambers 172 and 173communicate with each other through clearances between the bearings 162carrying the boss 52 a and a through-bore 174 provided in the boss 52 a.

[0140] When the oil is supplied from the oil pump 44 driven by theengine E through the oil passage 27 to the upstream supply oil passage27 a, the oil flows through the flow-in bore 170 into the oil chamber168, and is diverted therefrom into the through-bore 169 and theflow-out bore 171. The oil passed through the through-bore 169 issupplied friction portions and sliding portions of the shifting clutchCc′ to contribute to the cooling and lubrication of them.

[0141] On the other hand, the oil passed through the flow-out bore 171flows through the downstream section of the upper supply oil passage 27a and then through the flow-in bore 175 via the first small oil chamber172 to fill the oil chamber defined between the pump impeller 50 and theturbine impeller 51. Then, the oil flows from the oil chamber via thesecond small oil chamber 173 and the flow-out bore 176 to the downstreamsupply oil passage 27 b to lubricate the various portions of the engineE.

[0142] The boss 52 a of the stator impeller 52 is carried on thecrankshaft 2 with the bearings 162 interposed therebetween and hence, astable rotation is ensured. Moreover, opposite ends of the bearings 162face the first and second small oil chambers 172 and 173 on the oppositesides of the boss 52 a and hence, the bearings 162 can be always put ina well-lubricated state. The first and second small oil chambers 172 and173 communicate with each other through the bearings 162 and thethrough-bore 174 and hence, when the amount of oil supplied from the oilpump 44 is small, a large amount of the oil from the upstream supply oilpassage 27 a to the first small oil chamber 172 is insufficient when thepump impeller 50 intends to draw thereinto by the rotation. However, theoil flows from the second small oil chamber 173 through the through-bore174 and the bearings 162 into the first small oil chamber 172 tocompensate for such insufficiency and hence, it is possible to inhibitthe generation of air bubbles in the oil within the torque converter T′,to prevent the reduction in transmitting efficiency and to lubricate thebearings 162 effectively.

[0143] The first and second oil chambers 172 and 173 may communicatewith each other around the bearings 162 through a through-bore 174′, orthrough both of the bearings 162 and the through-bore 174′.

[0144] The direct communication between the upstream supply oil passage27 a and the downstream supply oil passage 27 b within the crankshaft 2is cut off by the partition wall 165 between the flow-in bore 175 andthe flow-out bore 176. Therefore, the oil supplied from the oil pump 44to the upstream supply oil passage 27 a is forcibly passed within thetorque converter T′ through the flow-in bore 175 and the flow-out bore176, and even if the oil pump 44 is of a relatively small capacity, itis possible to prevent the insufficiency of the operating oil in thetorque converter T′ to the utmost, which is effective for thesmall-sized vehicle.

[0145] Another arrangement is substantially the same as in the firstembodiment, and hence, portions or components corresponding to those inthe first embodiment are designated by like reference characters inFIGS. 15 to 17, and the description of them is omitted.

[0146] Although the embodiments of the present invention have beendescribed in detail, it will be understood that the present invention isnot limited to the above-described embodiments, and variousmodifications in design may be made without departing from the spiritand scope of the invention defined in claims. For example, the shiftingclutch Cc, Cc′ is disposed between the engine E and the torque converterT, T′ on the transmitting path between the engine E and the primaryreducing device 14 in the above-described embodiment, but may bedisposed between the torque converter T, T′ and the primary reducingdevice 14. In addition, the torque converter T, T′ may be replaced by afluid coupling which does not have a torque amplifying function.

What is claimed is:
 1. A transmitting system for a small-sized vehiclein which a crankshaft of an engine and an input shaft of a multi-stagetransmission are connected to each other through a fluid transmittingmeans including a pump impeller connected to the engine, and a turbineimpeller connected to said multi-stage transmission, wherein saidtransmitting system includes a shifting clutch which is interposedbetween said crankshaft of the engine and said input shaft of saidmulti-stage transmission, said shifting clutch being in a seriesrelation to said fluid transmitting means, and a lock-up clutch which isinterposed between said pump impeller and said turbine impeller of saidfluid transmitting means, said lock-up clutch being capable ofconnecting both said impellers directly to each other.
 2. A transmittingsystem for a small-sized vehicle according to claim 1 , wherein saidlock-up clutch comprises a pump extension connected to said pumpimpeller and surrounding said turbine impeller, a pressure receivingplate coupled to a tip end of said pump extension to define, within saidpump extension, a hydraulic pressure chamber communicating with an oilchamber defined between said pump impeller and said turbine impeller, apressing plate opposed to said pressure receiving plate and biasedtoward said pressure receiving plate by a hydraulic pressure in saidhydraulic pressure chamber, an annular friction clutch plate interposedbetween said pressure receiving plate and said pressing plate andconnected to said turbine impeller, first and second valve boresprovided in said pressing plate and said pressure receiving plate on theside of an inner periphery of said friction clutch plate, respectively,a control valve provided in said pressing plate to close said firstvalve bore, a control rod which is received in said first and secondvalve bores and movable between a retracted position in which saidcontrol rod causes the inner periphery of said friction clutch plate tobe open outside said second valve bore, while permitting the closing ofsaid control valve, and an advanced position in which said control rodcauses said control valve to be open to permit the inner periphery ofsaid friction clutch plate to communicate with said hydraulic pressurechamber, while closing said second valve bore, and an operating meansfor operating said control rod.
 3. A transmitting system for asmall-sized vehicle according to claim 1 or 2 , further including aone-way clutch which is interposed between said crankshaft and saidturbine impeller and brought into an ON state, when said turbineimpeller receives a reverse load torque.
 4. A transmitting system for asmall-sized vehicle according to claim 1 , wherein said lock-up clutchis constructed such that it depends on the rotational speed of the pumpimpeller and it automatically operates, when the rotational speed of thepump impeller becomes equal to or higher than a predetermined value. 5.A transmitting system for a small-sized vehicle according to claim 4 ,wherein said lock-up clutch comprises a pump extension connected to saidpump impeller and surrounding said turbine impeller, a pressurereceiving plate coupled to a tip end of said pump extension to define,within said pump extension, a hydraulic pressure chamber communicatingwith an oil chamber defined between said pump impeller and said turbineimpeller, a pressing plate opposed to said pressure receiving plate foradvancing and retracting movements, an annular friction clutch plateinterposed between said pressure receiving plate and said pressing plateand connected to said turbine impeller, a return spring for biasing saidpressing plate in a direction to be retracted relative to said pressurereceiving plate, and an escape bore which permits communication betweenthe inside and outside of said pressure receiving plate on an innerperipheral side of said friction clutch plate, wherein when therotational speed of said pump impeller becomes equal to or higher thanthe predetermined value, said pressing plate clamps said friction clutchplate in cooperation with said pressure receiving plate under the actionof a centrifugal hydraulic pressure which is raised in said hydraulicpressure chamber in accordance with the rotational speed of said pumpimpeller.
 6. A transmitting system for a small-sized vehicle accordingto claim 1 , wherein said lock-up clutch is constructed such that itdepends on the rotational speed of the turbine impeller and it operatesautomatically when the rotational speed of the turbine impeller becomesequal to or higher than a predetermined value.
 7. A transmitting systemfor a small-sized vehicle according to claim 6 , wherein said lock-upclutch comprises a clutch cylinder connected to said turbine impeller, apressing piston slidably received in a cylinder bore in said clutchcylinder to define a hydraulic pressure chamber, a piston return springfor biasing said pressing piston toward said hydraulic pressure chamber,a means for introducing oil into said hydraulic pressure chamber, and afriction engagement means provided between said clutch cylinder and saidpump impeller, wherein when the rotational speed of the turbine impellerbecomes equal to or higher than the predetermined value, said pressingpiston operates said friction engagement means under the action of acentrifugal hydraulic pressure risen within said hydraulic pressurechamber which is raised in accordance with the rotational speed of theturbine impeller to connect said clutch cylinder and said pump impellerdirectly to each other.
 8. A transmitting system for a small-sizedvehicle according to claim 7 , wherein said clutch cylinder is providedwith an escape bore which opens an outer periphery of said hydraulicpressure chamber to the outside, and a centrifugal valve which isadapted to open said escape bore, when the rotational speed of saidclutch cylinder is lower than a predetermined value, and to close saidescape bore, when the rotational speed of said clutch cylinder is equalto or higher than the predetermined value.